(1) Field of the Invention
The present invention relates to a method for crosslinking an isoprene-isobutylene rubber (hereinafter referred to as butyl rubber), a particular ethylene-propylene-diene rubber (ethylene-propylene-diene rubber is hereinafter referred to as EPDM), i.e. an ethylene-propylene-diene rubber containing ethylidenenorbornene as an unsaturated component (this rubber is hereinafter referred to as ENB type EPDM), or a mixture of a butyl rubber and an ENB type EPDM, using an alkylphenol-formaldehyde resin and a particular triazole compound; as well as to a crosslinked rubber product obtained by the above crosslinking method.
The present invention relates further to a method for crosslinking a butyl rubber, an ENB type EPDM or a mixture of a butyl rubber and an ENB type EPDM, using an alkylphenol-formaldehyde resin, a particular triazole compound and a hydrazide compound; as well as to a crosslinked rubber product obtained by the above crosslinking method.
(2) Description of the Prior Art
Butyl rubber is a known synthetic rubber produced by copolymerization of isoprene and isobutylene, having an unsaturation degree of 0.5 to 3.0 mole %. Crosslinked butyl rubber has low gas permeability, weather resistance, electrical insulation, heat resistance, damping property, resistance to acids and alkalis, low water absorption, etc. and is in use in rubber stopper, o-ring, packing, curing bag, tank lining, coating of electric wire, hose, automobile tube, rubber vibration insulator, etc.
For crosslinking of butyl rubber, there have been known three methods, i.e. sulfur crosslinking, quinoid crosslinking and resin crosslinking.
Sulfur crosslinking is conducted using sulfur together with a crosslinking accelerator such as thiuram, thiazole or the like, and is in wide use when a butyl rubber is produced into a rubber hose, a rubber vibration insulator, a tube for bicycle or automobile, etc.
Quinoid crosslinking is conducted using quinone dioxime or benzoylquinone dioxime together with red lead or lead dioxide, and is suited for continuous crosslinking of coated electric wire in which a high crosslinking rate is required.
Resin crosslinking is conducted using an alkylphenol-formaldehyde resin together with an inorganic halogen compound (e.g. tin chloride or ferrous chloride) or a halogen-containing elastomer (e.g. chloroprene rubber or chlorosulfonated polyethylene), or using a halogenated alkylphenol-formaldehyde resin, and is suited when a butyl rubber is produced into, for example, a curing bag used in tire production. This resin crosslinking can produce a crosslinked butyl rubber of low compression set, but corrodes the mold used because a halogen compound is used in the resin crosslinking.
EPDM is a terpolymer of ethylene, propylene and a diene and, as the third component diene, there is used 1,4-hexadiene, ethylidenenorbornene or the like. In the present invention, there is used an EPDM containing ethylidenenorbornene as an unsaturated component, i.e. an ENB type EPDM.
ENB type EPDM is a known synthetic rubber superior in heat resistance, electrical insulation and weather resistance. For crosslinking of ENB type EPDM, there are known four methods, i.e. organic peroxide crosslinking, sulfur crosslinking, quinoid crosslinking and resin crosslinking.
Sulfur crosslinking, similarly to the sulfur crosslinking for butyl rubber, is conducted using sulfur together with a crosslinking accelerator such as thiuram, thiazole or the like and gives a cured product having superior weather resistance. Therefore, this sulfur crosslinking is in wide use when gaskets for automobile or construction, roofing sheets, etc. are produced.
Organic peroxide crosslinking can give a cured product of low compression set and therefore is in wide use when industrial rubber products such as packing and the like are produced.
Quinoid crosslinking is conducted using quinone dioxime or benzoylquinone dioxime together with red lead or lead dioxide. This crosslinking is introduced in books regarding rubber technology, but is in substantially no use in industrial production of an ENB type EPDM into a rubber product.
Resin crosslinking is conducted using an alkylphenol-formaldehyde resin together with an inorganic halogen compound (e.g. tin chloride or ferrous chloride) or a halogen-containing elastomer (e.g. chloroprene rubber or chlorosulfonated polyethylene), or using a halogenated alkylphenol-formaldehyde resin. However, rubber products obtained by subjecting an ENB type EPDM to this resin crosslinking using a halogen compound together, find substantially no application.
In resin crosslinking of a butyl rubber or an ENB type EPDM is conducted using an alkylphenol-formaldehyde resin, an organic or inorganic halogen compound is used together. This resin crosslinking using a halogen compound, however, corrodes the mold used and finds substantially no practical application. When a butyl rubber or an ENB type EPDM is crosslinked using an alkylphenol-formaldehyde resin alone and no halogen compound, the crosslinking rate is low.
Crosslinking of, in particular, ENB type EPDM using an alkylphenol-formaldehyde resin alone is unstable. When such crosslinking is tried for a compound containing a large amount of clay as a filler, the crosslinking proceeds very slowly or no crosslinking takes place, making it impossible to produce a crosslinked rubber product.
Butyl rubber and ENB type EPDM are compatible with each other and can be mixed at any proportions. Therefore, they can be crosslinked (co-crosslinked) using a common crosslinking agent and a cured product retaining the properties of the individual rubbers can be obtained. However, a mixture of a butyl rubber and an ENB type EPDM is crosslinked using an alkylphenol-formaldehyde resin alone and no halogen compound, the ENB type EPDM is not sufficiently crosslinked; therefore, it is impossible to obtain a rubber product having good crosslinked rubber properties.
Mixing of two different rubbers is generally conducted by those skilled in the art, in order to impart the properties of one rubber into other rubber. In this case, use of a common crosslinking agent is a common knowledge.
In crosslinking a mixture of two different rubbers using a crosslinking agent which is effective only to either one rubber, the effect of the crosslinking agent to the other rubber need be considered and the properties of the crosslinked rubber obtained are difficult to predict in many cases. Therefore, such crosslinking is hard to employ on an industrial scale.
When a mixture of a butyl rubber and an ENB type EPDM is co-crosslinked by those skilled in the art, it is generally conducted by sulfur crosslinking. This sulfur crosslinking is effective, for example, when an ENB type EPDM is mixed into a butyl rubber hose or when a butyl rubber is mixed into an ENB type EPDM-made roofing sheet to improve the adhesivity of the ENB type EPDM, and can prevent heat-softening of butyl rubber during sulfur crosslinking; however, the sulfur crosslinking is unsuitable when the crosslinked rubber is used for applications such as packing (which requires a low compression set) and the like.
Co-crosslinking of a mixture of a butyl rubber and an ENB type EPDM using a phenolic resin alone and no halogen compound has little practical applicability, because the ENB type EPDM is not crosslinked often and the cured product obtained has no intended properties.
By mixing a butyl rubber into an ENB type EPDM at an ENB type EPFM proportion of at least 50%, the properties of the butyl rubber can be imparted to the ENB type EPDM. That is, improvements are obtained in the adhesion between crosslinked and uncrosslinked rubbers, reduction in gas permeability, flow of compound during rubber molding, and tear strength at high temperatures, correspondingly to the mixing ratio of the two rubbers. This is a specific case in which the adhesivity, low gas permeability and excellent tear strength at high temperatures, possessed by the butyl rubber are imparted to the ENB type EPDM.
By mixing an ENB type EPDM into a butyl rubber at a butyl rubber proportion of at least 50%, the properties of the ENB type EPDM can be imparted to the butyl rubber. Specifically, the mixture can have a sufficient hardness and can be free from softening, at ambient temperatures of 80xc2x0 C. or more; and an uncured rubber compound free from sticking can be obtained.
In producing those packings requiring steam resistance, water resistance, alkali resistance, acid resistance and heat resistance, a crosslinked rubber is usable which is obtained by crosslinking a butyl rubber, an ENB type EPDM or their mixture. This crosslinked rubber must also have a low compression set. As rubber products for which such properties are required, there are mentioned, for example, a packing for high-pressure steam sterilizer, a packing used at the joint of a high-pressure steam pipe, and a sealing rubber for capacitor. These rubber products, when used in contact with a metal, must also have no corrosivity to the metal, as an important property.
Sulfur crosslinking, however, is unable to give a crosslinked rubber product having an excellent (very low) compression set. In addition, the crosslinked rubber product obtained by sulfur crosslinking, when in contact with a metal (e.g. copper) reactive with sulfur, corrodes the surface of the metal.
Crosslinking using an organic peroxide can crosslink ENB type EPDM, but softens butyl rubber.
Quinoid crosslinking is usable for crosslinking of butyl rubber, ENB type EPDM or their mixture. However, this crosslinking is not preferred because it tends to cause early crosslinking (scorching) and the lead compound used together with quinoid has a problem of environmental pollution.
Resin crosslinking uses a halogen compound as a crosslinking co-agent and can give a crosslinked rubber superior in heat resistance and compression set. In this crosslinking, however, the mold used is corroded and/or stained by the halogen compound used as a crosslinking co-agent. Therefore, the resin crosslinking is unsuitable when a rubber product such as packing or the like is produced. Butyl rubber can be crosslinked using an alkylphenol-formaldehyde resin alone and no halogen compound; however, the crosslinking rate is low. Moreover, with such crosslinking, it is impossible to produce a butyl rubber of high hardness or a crosslinked butyl rubber of high modulus. Meanwhile, crosslinking of ENB type EPDM using an alkylphenol-formaldehyde resin alone and no halogen compound is unstable and no crosslinking takes place often. Therefore, it is very difficult to produce a rubber product of practical usability by crosslinking an ENB type EPDM using an alkylphenol-formaldehyde resin alone and no halogen compound. Hence, if resin crosslinking requiring no halogen compound is developed, it becomes possible to produce, from a butyl rubber or an ENB type EPDM or their mixture of any proportions, a rubber product not corrosive to metals. It further becomes possible to produce a novel rubber product by mixing the above two rubbers and crosslinking the mixture according to the crosslinking method of the present invention.
The first object of the present invention is to provide a novel crosslinking method which comprises adding, to a butyl rubber, an ethylene-propylene-diene rubber containing ethylidenenorbornene as an unsaturated component (an ENB type EPDM), or a mixture of desired proportions of a butyl rubber and an ENB type EPDM, an alkylphenol-formaldehyde resin and a particular triazole compound, wherein no halogen compound is added.
The second object of the present invention is to provide a novel crosslinking method which comprises adding, to a butyl rubber, an ENB type EPDM, or a mixture of desired proportions of a butyl rubber and an ENB type EPDM, an alkylphenol-formaldehyde resin, a particular triazole compound and a hydrazide compound, wherein no halogen compound is added.
The third object of the present invention is to provide a method for phenolic resin crosslinking of a butyl rubber, an ENB type EPDM or their mixture, which can produce a crosslinked rubber without corroding the mold used in the crosslinking; and a crosslinked rubber product by the crosslinking method, which does not corrode metals such as copper alloy, aluminum and the like when used in contact with such metals and which has a high hardness and a good (low) compression set.
Other objects of the present invention will become apparent from the following description.
The above objects of the present invention are achieved by a method for crosslinking a butyl rubber, an ENB type EPDM or a mixture of desired proportions of a butyl rubber and an ENB type EPDM, using an alkylphenol-formaldehyde resin and 3-(N-salicyloyl)amino-1,2,4-triazole; and a crosslinked rubber product produced by the above crosslinking method.
The objects of the present invention are also achieved by a method for crosslinking a butyl rubber, an ENB type EPDM or a mixture of desired proportions of a butyl rubber and an ENB type EPDM, using an alkylphenol-formaldehyde resin, 3-(N-salicyloyl)amino-1,2,4-triazole and a hydrazide compound; and a crosslinked rubber product produced by the above crosslinking method.
The present invention is described in detail below.
As mentioned above, the butyl rubber used in the present invention is a known synthetic rubber produced by copolymerization of isoprene and isobutylene, having an unsaturation degree of 0.5 to 3.0 mole %. The butyl rubber of the present invention does not include any halogenated butyl rubber which is a chlorine- or bromine-added isoprene-isobutylene rubber.
The particular ethylene-propylene-diene rubber used in the present invention, that is, an ethylene-propylene-diene rubber containing ethylidenenorbornene as an unsaturated component (a diene component) (an ENB type EPDM) is an ethylene-propylene-diene terpolymer containing ethylidenenorbornene (ENB) as a diene component. The ENB type EPDM used in the present invention is a known synthetic rubber which is produced by copolymerizing ethylene, propylene and ethylidenenorbornene using a vanadium-based catalyst, an organoaluminum type catalyst, a metallocene catalyst or the like and which contains ethylidenenorbornene as the third component in an amount of generally 5 to 30 mole % (2 to 16 wt. %) as calculated from the iodine value and propylene in an amount of 8 to 50 mole %.
When a butyl rubber is allowed to have the properties of ENB type EPDM or when an ENB type EPDM is allowed to have the properties of butyl rubber, a butyl rubber and an ENB type EPDM may be mixed at appropriate proportions depending upon the intended purpose, and there is no restriction as to the mixing proportions. For example, in order to reduce the hardness of a butyl rubber at atmospheres of xc2x0 C. or higher, an ENB type EPDM may be mixed into the butyl rubber at a proportion of about 5 to 30% by weight; in order to improve the tear strength of an ENB type EPDM at high temperatures of 170xc2x0 C. or more (encountered during molding of a product of complicated shape using a mold), a butyl rubber may be mixed into the ENB type EPDM at a proportion of about 5 to 30% by weight. Thus, the mixing ratio can be varied easily.
The alkylphenol-formaldehyde resin used in the present invention is not critical and may be any alkylphenol-formaldehyde resin as long as it can be effectively used in the resin crosslinking of the present invention for butyl rubber, ENB type EPDM or their mixture. However, a methylol group-containing compound of relatively low molecular weight is preferred. There is preferred, for example, a mixture of low-molecular compounds each represented by the following general formula (1): 
wherein n is 0 to 10, R is an aliphatic group having 1 to 10 carbon atoms, and Rxe2x80x2 is xe2x80x94CH2xe2x80x94 or xe2x80x94CH2OCH2xe2x80x94. Such compounds are commercially available as, for example, TACKIROL 201 (a product of Taoka Chemical Co., Ltd.) and HITANOL 2501 (a product of Hitachi Chemical Co., Ltd.).
The amount of the alkylphenol-formaldehyde resin added is 8 to 25 parts by weight, preferably 10 to 20 parts by weight per 100 parts by weight of the synthetic rubber component which is a butyl rubber, an ENB type EPDM or a mixture of desired proportions of a butyl rubber and an ENB type EPDM. When the amount is less than 8 parts by weight, no intended effect is obtained. When the amount is more than 25 parts by weight, the raw material compound obtained is very sticky, resulting in reduced operability. The objects of the present invention are achieved at the alkylphenol-formaldehyde resin amount of 25 parts by weight or less.
Use of a halogenated alkylphenol-formaldehyde resin wherein the methylol group or the benzene ring is substituted with halogen (e.g. bromine), is unsuitable for the objects of the present invention and is not included in the scope of the present invention.
The 3-(N-salicyloyl)amino-1,2,4-triazole used in the present invention is represented by the following formula (2). 
The amount of 3-(N-salicyloyl)-1,2,3-triazole used is 0.1 to 8 parts by weight, preferably 0.2 to 5 parts by weight per 100 parts by weight of the synthetic rubber component which is a butyl rubber, an ENB type EPDM or a mixture of desired proportions of a butyl rubber and an ENB type EPDM. When the amount is less than 0.1 part by weight, the addition effect is low. When the amount is more than 8 parts by weight, no additional effect is obtained and a sufficient effect can be obtained at an amount of 8 parts by weight or less.
In the present invention, it was found out that 3-(N-salicyloyl)amino-1,2,4-triazole exhibits a unique property of good crosslinking acceleratability in the alkylphenol-formaldehyde resin crosslinking for butyl rubber or ENB type EPDM; and the finding has led to the completion of the present invention.
3-Amino-1,2,4-triazole, which has a chemical structure very similar to that of 3-(N-salicyloyl)amino-1,2,4-triazole, shows no crosslinking acceleratability in the alkylphenol-formaldehyde resin crosslinking for butyl rubber or ENB type EPDM and causes foaming owing to the heat during crosslinking, making it impossible to obtain a crosslinked rubber. This indicates the unique property of the triazole compound of the present invention.
As the hydrazide compound usable in the present invention, there can be mentioned saturated or unsaturated aliphatic dibasic acid hydrazides; dibasic acid hydrazides having a hydantoin skeleton; phthalic acid hydrazides; hydrazide compounds obtained by a reaction of each one hydrogen atom of the two xe2x80x94NH2 possessed by the above hydrazide compounds, with benzoic acid, o-, m- or p-toluylic acid or o-, m- or p-oxybenzoic acid; carbohydrazide; and so forth.
As specific examples of the hydrazide compound, there can be mentioned carbohydrazide, adipic acid dihydrazide, sebacic acid dihydrazide, dodecanedioic acid dihydrazide, isophthalic acid hydrazide, maleic acid hydrazide, decamethylenedicarboxylic acid disalicyloylhydrazide, eicosanedioic acid dihydrazide, 7,11-octadecadiene-1,18-dicarbohydrazide, and 1,3-bis(hydrazinocarboethyl)-5-isopropylhydantoin.
By using, in combination, the triazole compound of the present invention and the above hydrazide compound, the crosslinking rate of the alkylphenol-formaldehyde resin can be increased further; production of, in particular, a rubber product of high hardness becomes possible without increasing the amount of carbon black (a filler) used; and a compound of good moldability can be obtained.
The amount of the hydrazide compound used is 0.1 to 5 parts by weight, preferably 0.3 to 4 parts by weight (when hydrazide compounds of two or more kinds are used, the amount is their total amount), per 100 parts by weight of the synthetic rubber component which is a butyl rubber, an ENB type EPDM or a mixture of desired proportions of a butyl rubber and an ENB type EPDM. When the amount of the hydrazide compound used is less than 0.1 part by weight, the addition effect is low. When the amount is more than 5 parts by weight, no additional effect is obtained.
The hydrazide compound used in the present invention is described in more detail below.
The saturated or unsaturated aliphatic dibasic acid hydrazides used in the present invention are (a) dibasic acid hydrazides derived from saturated or unsaturated aliphatic dicarboxylic acids and (b) hydrazide compounds obtained by a reaction of each one hydrogen atom of the two xe2x80x94NH2 possessed by the above dibasic acid hydrazides (a), with benzoic acid, o-, m- or p-toluylic acid or o-, m- or p-oxybenzoic acid, both the dibasic acid hydrazides (a) and the hydrazide compounds (b) being represented by the following general formula (3): 
[wherein X and Y may be the same or different and are each a hydrogen atom or an alkyl group having 1 to 5 carbon atoms; R is a hydrogen atom or a group represented by 
(wherein Rxe2x80x2 is a hydrogen atom, a methyl group or a hydroxyl group); n is a number of 0 to 2; and m is a number of 0 to 20 (n and m are not 0 simultaneously)]. In the formula (3), [ ] indicates that the bond 
and the bond 
are combined at random.
In the following formulas (4) to (6) are shown the structural formulas of sebacic acid hydrazide (which is a saturated aliphatic dibasic acid hydrazide), 7,11-octadecadiene-1,18-dicarboxyhydrazide (which is an unsaturated aliphatic dibasic acid hydrazide) and decamethylenedicarboxylic acid disalicyloylhydrazide (which is a reaction product of a saturated aliphatic dibasic acid hydrazide with o-oxybenzoic acid). 
(Decamethylenedicarboxylic acid disalicyloylhydrazide)
The dibasic acid hydrazides having a hydantoin skeleton and the hydrazide compounds obtained by a reaction of each one hydrogen atom of the two xe2x80x94NH2 possessed by the above dibasic acid hydrazides, with benzoic acid, o-, m- or p-toluylic acid or o-, m- or p-oxybenzoic acid are represented by the following general formula (7): 
wherein R is a hydrogen atom or an alkyl group having 1 to 10 carbon atoms; Rxe2x80x2 is a hydrogen atom or a group represented by 
(wherein Rxe2x80x3 is a hydrogen atom, a methyl group or a hydroxyl group); and n is a number of 1 to 10].
In the following formula (8) is shown the structural formula of 1,3-bis(hydrazinocarboethyl)-5-isopropylhydantoin which is a specific example of the compounds of the formula (7). 
The phthalic acid hydrazides are shown by the following general formula (9): 
wherein R has the same definition as Rxe2x80x2 of the general formula (7).
In the following formula (10) is shown the structural formula of isophthalic acid hydrazide which is a specific example of the compounds of the general formula (9). 
The carbohydrazide is represented by the following formula (11). 
When a butyl rubber, an ENB type EPDM or a mixture of desired proportions of a butyl rubber and an ENB type EPDM is thermally crosslinked using an alkylphenol-formaldehyde resin and a triazole compound, a higher crosslinking rate is obtained. When, in that crosslinking, a hydrazide compound is also used, (1) an even higher crosslinking rate is obtained; (2) release of the chlorine contained as an impurity in the additives (e.g. clay and carbon black) used, from the crosslinked rubber obtained can be prevented, enabling production of a good rubber product showing no corrosivity for a long time to metals (e.g. copper alloy and aluminum) corrodible by halogen compounds; and (3) the crosslinked rubber product can have an increased hardness without an increase in the amount of carbon black used. Further, according to the crosslinking method of the present invention, it is easy to produce a crosslinked rubber product having an excellent compression set and a hardness of 85 or more in terms of durometer hardness A, and a product (e.g. packing) produced using a mold is provided easily. The crosslinking method of the present invention can achieve the first, second and third objects of the present invention.
In the present invention, crosslinking acceleratability is confirmed by determining a crosslinking curve using an oscillating rheometer. Specifically, the property improvement of a crosslinked rubber according to the present invention can be confirmed by comparing the differences (increases) of hardness and modulus between the above crosslinked rubber and a crosslinked rubber having the same composition except that no crosslinking accelerator is contained. The increased hardness and modulus are very important properties of the crosslinked rubber obtained by the present invention.
The composition containing a butyl rubber, an ENB type EPDM or a mixture of desired proportions of a butyl rubber and an ENB type EPDM, a crosslinking agent and a crosslinking accelerator according to the present invention may further contain, as necessary, additives generally used in rubbers, such as carbon black, filler (e.g. kaolin, clay, mica or calcium carbonate), antioxidant, zinc white, stearic acid and the like.
The additives usable in the present invention are substances added as base materials for improvement in properties such as strength, processability, durability and the like, or substances added for volume or weight increase, cost reduction, etc. Specifically, they are additives generally used by those skilled in the art, such as carbon blacks (e.g. furnace black and thermal black), dry process white carbon, wet process white carbon, kaolinite (clay) or fired clay obtained by firing kaolinite (clay), silane-treated clay obtained by subjecting clay to a surface treatment with a silane coupling agent, calcium carbonate, zinc oxide, magnesium oxide and the like.
Besides, there may be also added, as necessary, an antioxidant, stearic acid, an AC polyethylene, a paraffin wax, a processing aid, etc.
In carrying out the crosslinking of the present invention, the crosslinking conditions are not critical. Ordinarily, however, primary crosslinking is conducted at 170 to 210xc2x0 C. for 5 to 20 minutes by the use of a hot press used by those skilled in the art. Secondary crosslinking is not necessary depending upon the product obtained; however, when the crosslinked rubber needs to have a superior compression set, a higher hardness, etc., secondary crosslinking is conducted at 170 to 210xc2x0 C. for 30 minutes to 3 hours.
According to the crosslinking method of the present invention, an accelerated crosslinking rate is obtained in the resin crosslinking of a butyl rubber, an ENB type EPDM or a mixture of desired proportions of a butyl rubber and an ENB type EPDM. As a result, a crosslinked rubber made from a butyl rubber, an ENB type EPDM or a mixture of desired proportions of a butyl rubber and an ENB type EPDM, having a hardness of 85 or more in terms of durometer hardness A, can be easily produced using no halogen compound as a crosslinking co-agent without reduction in electrical insulation; and a crosslinked rubber product of good moldability, low compression set and low corrosivity to metals can be obtained.
The present invention has a feature in that the 3-(N-salicyloyl)amino-1,2,4-triazole used functions as a good crosslinking accelerator in the resin crosslinking of a butyl rubber, an ENB type EPDM or a mixture of desired proportions of a butyl rubber and an ENB type EPDM. The present invention further has a feature in that the combined use of 3-(N-salicyloyl)amino-1,2,4-triazole and a hydrazide compound enables (1) an even higher crosslinking rate, (2) prevention of release of chlorine (contained as impurities in additives such as clay, carbon black and the like) from crosslinked rubber product, and (3) production of crosslinked rubber product of high hardness without increase in carbon black amount used. In the crosslinking method of the present invention, since a butyl rubber and an ENB type EPDM can be mixed in any desired ratio, it is possible to impart the properties of ENB type EPDM to a butyl rubber or the properties of butyl rubber to an ENB type EPDM; therefore, the heat resistance, gas permeability, hardness, tackiness, etc. of crosslinked rubber product can be easily controlled so as to meet the desired levels of these properties.